This invention relates to bar code reading.
In bar code readers, a light beam is scanned across a bar code symbol, and reflected light is collected, using either retroreflective or non-retroreflective light collection, and decoded into electrical signals representative of the information stored on the bar code symbol.
In retroreflective readers, a moving optical component, e.g., a rotating mirror, as described, e.g., in Krichever et al. U.S. Pat. No. 4,816,661 or Shepard et al. U.S. Pat. No. 4,409,470, both herein incorporated by reference, scans the beam across a target surface and directs the collected light to a detector. The rotating mirror must be relatively large to receive the incoming light, but only a small detector is required because the rotating mirror can focus the light onto a small area. As a result, the detector has a relatively small field-of-view, which increases the signal-to-noise ratio of the reader.
In non-retroreflective bar code readers, the reflected laser light is not collected by the same rotating mirror used for scanning. Instead, the detector is constructed to have a large field-of-view so that the reflected laser light traces across the surface of the detector. Because the rotating mirror need only handle the outgoing light beam, it can be made much smaller. But the field-of-view of the detector must be relatively large in order to receive the incoming light beam from all scanned locations.
In recent years, it has become increasingly desirable to increase the scan rate of bar code readers (e.g., to increase the throughput of scanned items). There has also been a push toward smaller, hand-held bar code readers. While the performance of hand-held readers has reached acceptable levels, the ability to improve the performance of such reader has been somewhat elusive. As bar code readers become smaller, the optics used to collect light reflected from the bar code symbol generally become proportionally smaller, and thus, the levels of the collected signals are becoming smaller, as well. However, noise in the electronic components of the reader does not scale proportionally with the optical collection area. This reduces the signal-to-noise ratio of the readers, which reduces their performance. For example, lower signal-to-noise ratios reduce the working range of the reader and increases the reader's susceptibility to ambient light corruption.